The invention relates generally to film cooling at high temperatures, and more specifically, to formation of trenches to improve film cooling.
A variety of components in aircraft engines and stationary power systems are operated in extremely hot environments. These components are exposed to hot gases having temperatures up to about 3800 degrees Fahrenheit, for aircraft applications, and up to about 2700 degrees Fahrenheit for stationary power generation applications. To cool the components exposed to the hot gases, these “hot gas path” components typically have both internal convection and external film cooling. For example, a number of cooling holes may extend from a relatively cool surface of the component to a “hot” surface of the component. Film cooling is of a higher benefit since it decreases incident heat flux from hot gases to surfaces of components.
The coolant typically is compressed air bled off a compressor, which is then bypassed around the engine's combustion zone and fed through the cooling holes to the hot surface. The coolant forms a protective “film” between the hot component surface and the hot gas flow, thereby helping protect the component from heating. Furthermore, protective coatings such as for example, thermal barrier coatings (TBCs) may be employed on the hot surface to increase operating temperature of the components. Film cooling is highest when the coolant flow hugs the hot surface. Hence, different surface geometries and shapes such as, but not limited to, trenches and craters, are formed in order to enable a longer duration of contact between the coolant flow and the hot surface and/or cooler effective gas temperature layer on the surface.
Laser drilling and electro-discharge machining (EDM) are commonly used techniques for forming film cooling holes. Film holes are currently drilled prior to or after the coatings are applied. Furthermore, various masking methods are generally employed to form the different surface geometries and shapes to improve film cooling effectiveness. However, the masking methods are not precise enough in terms of forming the geometries of predetermined dimensions and also result in deposition of the coatings like TBCs, into undesirable locations within the film holes.
Conventional lasers for cooling hole drilling use lasers with high pulse energy, around millisecond pulse duration, relatively low repetition rate (<1000 Hz), and the wavelength is typically 1064 nm or 10640 nm. Such laser processing results in a high drilling speed due to large pulse energy and high average power. However, it also results in a large heat affected zone, undesirable degree of delamination, and over drilling. On the other hand, shorter pulsed (<200 nanosecond) lasers are good for shallow structures (such as <500 micron features), but due to their low average power (<20 W) and low pulse energy (<1 mJ), and due to the specialty of film cooling holes (>2 mm in thickness, special applications in aviation etc.), these lasers have not been well developed into film cooling hole applications. Accordingly, existing laser systems mentioned above need further development to be both feasible and cost effective for desired applications.
Accordingly, there is a need for an improved laser technique to address one or more aforementioned issues.